1. Field of the Invention
This invention relates generally to torque limiter arrangements for rotating devices such as electric retractable door mirrors, and more particularly to torque limiter arrangements for rotating devices which can be rotated by external force.
2. Description of the Related Art
Electric door mirrors have torque limiting mechanisms on a way of power transmission devices of drive assemblies. The torque limiter prevents overload of a motor, damps shocks due to external force, and makes it possible to manually fold the mirror.
The door mirror rotates between a fixed return position and a fixed retracted position in response to rotation of the motor. The torque limiter is released when external force is applied to the door mirror by hands or the like, thereby enabling rotation of the mirror, whether or not the motor is driven.
In these door mirrors, a release torque of the torque limiter is set higher than a rotational resistance of a mirror body. However, the release torque decreases in long use of the torque limiter. Contrary thereto, the rotational resistance increases due to abrasion, rattles, dirt stick, etc., of the torque limiter. Thus, difference lessens between the release torque and the rotational resistance. When the torque becomes the same as the resistance, the drive assembly runs idle.
For example, Japanese Laid Open Patent Publication No. 61-12452, that is laid open to public inspection on Jan. 20, 1986, discloses a torque limiter arrangement used in an electric door mirror.
The arrangement has first and second torque limiters on a shaft of a base fixed to a car body. The first torque limiter has a predetermined rotational resistance or a first release torque between the fixed base and a rotating frame, to which a mirror is attached. The first torque limiter permits relative rotation between the base and the mirror when a torque not less than the first release torque is relatively applied to the base and the mirror. The second torque limiter has a fixed clutch on the shaft and a clutch gear drivingly coupled with a motor on the frame. The second torque limiter permits relative rotation between the fixed clutch and the clutch gear when a predetermined second release torque or more torque is relatively applied to the fixed clutch and the clutch gear.
There are provided a first set of balls and pockets, engageable with each other, around the shaft on opposite surfaces of the fixed clutch and the clutch gear. A second set of balls and pockets and similarly arranged on opposite surfaces of the base and the frame. A coil spring is disposed between the first and second torque limiters so as to urge each torque limiter and determine their release torques. Since the same urging force is given to each torque limiter, it is necessary to change an engaging depth of the ball in the pocket in the first set from that of the second set in order to obtain the difference between the release torques. However, it has a limitation to make the engaging depths different between the first set and the second set. Therefore, it is difficult to get the difference large between the release torques.
Moreover, in case the second limiter has been repeatedly used and has the edges of the pockets worn by the balls, the engaging depth is made smaller, thus decreasing the release torque thereof. When the release torque becomes the same as or smaller than the rotational resistance, the frame does not rotate relative to the base while the clutch gear is rotated relative to the fixed clutch. As a result, it is impossible to control rotation of the door mirror.